1. Field of the Invention
This invention relates to wind machines for converting wind energy into such forms that can be useful for different purposes, such as electric power, or pneumatic or hydraulic power accumulated in a fluid under pressure. In this respect it has its aim in common with the well-known windmills, which may produce electric power, or pump liquids, or do other useful work, according to what devices are connected to the shaft of the mill.
More specifically, the invention relates to such machines which have a vertical axis of rotation, and, still more specifically, to the kind of such machines in which there is found no stationary structures, nor a vertical shaft extending substantially above ground level.
2. Description of Prior Art
______________________________________ 665,810 Stretch 1,352,859 Wilson 1,791,731 Madarasz 1,835,018 Darrieus 4,302,684 Gogins 3,504,988 Stenner ______________________________________
In addition to the conventional type of windmills, whose somewhat twisted blades radiate from a hub on a horizontal shaft, (Horizontal Axis Wind Turbines, or HAWTs), there is known different types of wind machines with a vertical axis of rotation (or VAWTs). One of the kind was described by Wilson (U.S. Pat. No. 1,352,859), another by Darrieus (U.S. Pat. No. 1,835,018). At present, there is known several structures with a number of vertical airfoils supported by a system of horizontal spokes radiating from a hub which is supported on the top of a bearing structure, and which hub, in turn, transfers a torque from the airfoils to a vertical shaft.
With the structure described by Wilson as one exception, most VAWTs have airfoils which perform their circular movement with their foils maintained in a fixed position relative to the axis, but some of them include mechanical devices which make the foils deviate somewhat inwards and outwards from a tangential course, in order to decrease the torque while the rotor runs at a moderate speed relative to the wind velocity. Other precautions to increase the torque, or to facilitate the starting-up of the turbine, are known in different forms of VAWTs which have a real vertical shaft extending into the air.
In addition to these VAWTs, a few wind machines are suggested in which the vertical shaft is done away with. Instead, the structures on which the wind acts are mounted on a platform rotating on a system of wheels, or on a number of vehicles which perform a closed loop. Stretch (U.S. Pat. No. 665,810) described such a platform, which could be put in rotation by sails. According to J. D. Madarasz (U.S. Pat. No. 1,791,731), the structures collecting the wind forces may be rotating cylinders which utilize the Magnus effect to create a force oblique to the wind direction. Laird Gogins (U.S. Pat. No. 4,302,684) suggested a system of vertical airfoils supported on cars that travel around a fixed-loop track. If this track was chosen to be a circular one, instead of his specified elongated track, his system would have another feature in common with those of Stretch and Madarasz.
All of these wind machines have their draw-backs, even if most windmills perform to satisfaction the conversion of wind energy for the particular purpose they are built for. A most striking feature of all HAWTs is their inherent limitation of being scaled up for greater power production. This limitation exists because the area swept by the blades, and hence the power extracted from a wind of a certain velocity, increases with the second power of the linear measures, whereas the necessary strength of the supporting structure, and the moment of resistance of a cross section of it, as well as the quantity of construction material consumed for its erection, all increase with the third power of the linear measures. Consequently, the cost of building such a structure increases more rapidly than the energy it will convert, when the linear measures are increased. Even with the use of modern materials of construction, and by applying modern aerodynamic theory and experiences in their construction, there is an economical limit for the size of the HAWTs. Much the same is true for the VAWTs, even if more moderate quantities of construction materials, and pieces of equipment, are put aloft by their erection, because most mechanical and electrical devices may be kept at ground level. In general, the VAWTs now known can not, either, to an advantage, be scaled up beyond a certain limit.
Aerodynamically, both types operate most efficiently with a certain ratio of their vanes' tip speed to the wind velocity, and consequently they tend to speed up with increasing wind velocity. If they are coupled to an electrical AC generator which, in turn, is connected to an extended network, their speed of rotation have to be constant, if any intricate and expensive pieces of equipment, mechanical or electrical, are not added to the system, to allow for a speed variation. Another precaution sometimes found necessary has been to install brakes to take up surplus energy in heavy winds, and in other cases the windmill has to be put to a standstill when the wind velocity exceeds a certain limit.
Regarding the VAWTs in which the vertical shaft is done away with, they have in common the feature of one or more electric generator(s) being carried around with the rotating platform or with the vehicles, while they are driven by the wheels on which the structure run. As a consequence of this feature, the electric current produced by the generators have to be transferred to the external network through moving contacts, which has to be considered as an inherent drawback of these systems.